Do Alien Invasive Species Affect Frogs?
The answer is, yes! Just like many other native fauna, native amphibians (frogs, newts and salamanders) suffer huge losses due to the presence of alien invasive species. Invasive species occupy habitats outside of their home range, hence called alien, and increase the economic and environmental costs. Alien invasive species multiply rapidly in the absence of predators in their new home. They damage crops, use up resources meant for native species, change community structure by altering prey and predator relationships, transfer several pathogens and destroy people’s livelihoods by not just impacting agriculture but also by damaging several other ecosystems. Globally, the world spends about $1.2 trillion for managing invasives. Scientists across the world have said that simple steps towards prevention of spread of invasive species can save trillions of dollars.
Across numerous developed countries from the Global North, studies have shown that alien, non-native fish like trouts e.g. Oncorynchus mykiss, bass Micropterus spp., Amur sleeper fish Perccottus glenii, bluegill Lepomis macrochirus and mosquitofish Gambusia affinis or G. holbrooki, impact the presence of native amphibians. Manmade, permanent water bodies house a number of non-native fish. Such fish are often seen around urban and suburban areas. Apart from beautification and landscaping, non-native fish do not serve any other purposes within urban waterbodies. Some alien fish, however, are also introduced for game fishing, cultivation and for the reduction of mosquito populations, e.g. mosquitofish Gambusia affinis.
Alien fish use up resources like space and food that are essential for the survival and reproduction of native and endemic amphibians. Some of the fish are also known to feed on amphibian larvae and tadpoles. Not just due to predation pressure, some amphibian species simply avoid the habitats used by alien species, reducing habitat availability for them. This puts unnecessary pressure on the existing amphibian populations. At times, the number of fish is so high that the fish end up wiping out newly hatched populations of amphibians, thus leaving very few animals to grow up into adults and complete their life cycle. This causes local amphibian population extinction which eventually adds up to the existing amphibian decline.
Amphibian populations have declined by almost 80% in the last four decades. Land use changes like construction and beautification of existing freshwater habitats and the presence of alien invasive species are some of the leading causes of amphibian decline. About 88% of ‘Threatened amphibians’ are impacted by habitat changes, like habitat loss and degradation. These declines are far more than other species like birds or mammals. Some of the reasons for such rapid amphibian decline is their sensitivity to change. Given their permeable skin, inability to disperse longer distances, unlike birds and mammals, and specific needs for reproduction — amphibians are at a losing end with just the slightest of disturbances. As urban sprawl and agricultural land use increases, existing dispersal corridors are disrupted and wetlands are degraded. Therefore, comparative analysis between the declines of frogs, birds and mammals show that a total of 21 % of amphibians are categorised as ‘Critically Endangered’ and ‘Endangered’ while only 5.4 % of mammals and about 10.5 % of birds fall into these categories.
Declining numbers across amphibian populations suggests that amphibians are indeed sensitive to habitat change and alien species. However, not all amphibians are sensitive. Some amphibians themselves are invasive and add to the native amphibian population decline. The American bullfrog Lithobates catesbeianus, cane toad Rhinella marina and the African clawed frog Xenopus laevis often benefit from human made land use and wetland changes. Most of these frogs feed on insects attracted by increased urban lighting and make permanent water bodies their breeding grounds. They also extend their home ranges and invade breeding grounds and spaces with native amphibians.
Research shows that alien frogs reduce the growth and development of native leopard frogs and spotted salamanders in Florida. Alien frogs also feed on the eggs, tadpoles and adults of native frogs. The native Indian bullfrog Hoplobatrachus tigerinus is termed as invasive in Andaman archipelago and is known to impact native amphibian diversity by feeding on tadpoles. The African clawed frog is known to spread two deadly pathogens called the Chytrid fungus and Ranavirus in newer habitats that it occupies. Alien frogs also imbalance the community structure by adding in more predators and preying on available insects. Thus, native amphibian fauna suffers with less availability of food, space and breeding grounds, along with the threat of diseases and increased predation from the alien species.
An estimated 16% of the total amphibians on this planet are threatened by introduced species alone. Habitat restoration and maintenance within urban areas must remove and limit the use of exotic species. Facilitating connectivity within habitats and managing wetlands according to native species’ requirements is key to saving millions of dollars and conserving biodiversity.
This article was first published in ENVIS newsletter Jan 2023. Read here https://intranet.cb.amrita.edu/download/public/2022/csd/newsletter/Bioinvasion-January.pdf
Author Info:
Madhushri Mudke
PhD candidate and ZSL-EDGE fellow
&
Aravind Neelavar Ananthram, PhD, FLS
Convenor and Senior Fellow (Associate Professor)
SM Sehgal Foundation Center for Biodiversity and Conservation,
Ashoka Trust for Research in Ecology and the Environment (ATREE)
Manipal Academy of Higher Education (MAHE)
